A Sentimental Education: Sentiment Analysis Using Subjectivity Bo Pang Abstract

A Sentimental Education: Sentiment Analysis Using Subjectivity Bo Pang Abstract
A Sentimental Education: Sentiment Analysis Using Subjectivity
Summarization Based on Minimum Cuts
Bo Pang and Lillian Lee
Department of Computer Science
Cornell University
Ithaca, NY 14853-7501
pabo,llee @cs.cornell.edu
Sentiment analysis seeks to identify the viewpoint(s) underlying a text span; an example application is classifying a movie review as “thumbs up”
or “thumbs down”. To determine this sentiment polarity, we propose a novel machine-learning method
that applies text-categorization techniques to just
the subjective portions of the document. Extracting
these portions can be implemented using efficient
techniques for finding minimum cuts in graphs; this
greatly facilitates incorporation of cross-sentence
contextual constraints.
Publication info: Proceedings of the ACL, 2004.
1 Introduction
The computational treatment of opinion, sentiment,
and subjectivity has recently attracted a great deal
of attention (see references), in part because of its
potential applications. For instance, informationextraction and question-answering systems could
flag statements and queries regarding opinions
rather than facts (Cardie et al., 2003). Also, it
has proven useful for companies, recommender systems, and editorial sites to create summaries of people’s experiences and opinions that consist of subjective expressions extracted from reviews (as is
commonly done in movie ads) or even just a review’s polarity — positive (“thumbs up”) or negative (“thumbs down”).
Document polarity classification poses a significant challenge to data-driven methods, resisting traditional text-categorization techniques (Pang, Lee,
and Vaithyanathan, 2002). Previous approaches focused on selecting indicative lexical features (e.g.,
the word “good”), classifying a document according to the number of such features that occur anywhere within it. In contrast, we propose the following process: (1) label the sentences in the document
as either subjective or objective, discarding the lat-
ter; and then (2) apply a standard machine-learning
classifier to the resulting extract. This can prevent
the polarity classifier from considering irrelevant or
even potentially misleading text: for example, although the sentence “The protagonist tries to protect her good name” contains the word “good”, it
tells us nothing about the author’s opinion and in
fact could well be embedded in a negative movie
review. Also, as mentioned above, subjectivity extracts can be provided to users as a summary of the
sentiment-oriented content of the document.
Our results show that the subjectivity extracts
we create accurately represent the sentiment information of the originating documents in a much
more compact form: depending on choice of downstream polarity classifier, we can achieve highly statistically significant improvement (from 82.8% to
86.4%) or maintain the same level of performance
for the polarity classification task while retaining
only 60% of the reviews’ words. Also, we explore extraction methods based on a minimum cut
formulation, which provides an efficient, intuitive,
and effective means for integrating inter-sentencelevel contextual information with traditional bag-ofwords features.
2 Method
One can consider document-level polarity classification to be just a special (more difficult) case
of text categorization with sentiment- rather than
topic-based categories. Hence, standard machinelearning classification techniques, such as support
vector machines (SVMs), can be applied to the entire documents themselves, as was done by Pang,
Lee, and Vaithyanathan (2002). We refer to such
classification techniques as default polarity classifiers.
However, as noted above, we may be able to im-
prove polarity classification by removing objective
sentences (such as plot summaries in a movie review). We therefore propose, as depicted in Figure
1, to first employ a subjectivity detector that determines whether each sentence is subjective or not:
discarding the objective ones creates an extract that
should better represent a review’s subjective content
to a default polarity classifier.
m-sentence extract
positive or negative
n-sentence review
subjectivity extraction
Figure 1: Polarity classification via subjectivity detection.
To our knowledge, previous work has not integrated sentence-level subjectivity detection with
document-level sentiment polarity. Yu and Hatzivassiloglou (2003) provide methods for sentencelevel analysis and for determining whether a document is subjective or not, but do not combine these
two types of algorithms or consider document polarity classification. The motivation behind the singlesentence selection method of Beineke et al. (2004)
is to reveal a document’s sentiment polarity, but they
do not evaluate the polarity-classification accuracy
that results.
Context and Subjectivity Detection
As with document-level polarity classification, we
could perform subjectivity detection on individual
sentences by applying a standard classification algorithm on each sentence in isolation. However, modeling proximity relationships between sentences
would enable us to leverage coherence: text spans
occurring near each other (within discourse boundaries) may share the same subjectivity status, other
things being equal (Wiebe, 1994).
We would therefore like to supply our algorithms
with pair-wise interaction information, e.g., to specify that two particular sentences should ideally receive the same subjectivity label but not state which
label this should be. Incorporating such information is somewhat unnatural for classifiers whose input consists simply of individual feature vectors,
such as Naive Bayes or SVMs, precisely because
such classifiers label each test item in isolation.
One could define synthetic features or feature vec-
tors to attempt to overcome this obstacle. However,
we propose an alternative that avoids the need for
such feature engineering: we use an efficient and
intuitive graph-based formulation relying on finding minimum cuts. Our approach is inspired by
Blum and Chawla (2001), although they focused on
similarity between items (the motivation being to
combine labeled and unlabeled data), whereas we
are concerned with physical proximity between the
items to be classified; indeed, in computer vision,
modeling proximity information via graph cuts has
led to very effective classification (Boykov, Veksler,
and Zabih, 1999).
2.3 Cut-based classification
Figure 2 shows a worked example of the concepts
in this section.
Suppose we have items to divide
into two classes and , and we have access to
two types of information:
Individual scores : non-negative estimates of each ’s preference for being in based
on just the features of alone; and
Association scores "!#!$%&'
(& : non-negative
estimates of how important it is that ) and ( be in
the same class.1
We would like to maximize each item’s “net happiness”: its individual score for the class it is assigned to, minus its individual score for the other
class. But, we also want to penalize putting tightlyassociated items into different classes. Thus, after
some algebra, we arrive at the following optimization problem: assign the s to and so as to
minimize the partition cost
+ ,-.
+&, -54
+98:,&- .';
The problem appears intractable, since there are
possible binary partitions of the ) ’s. However, suppose we represent the situation in the following manner. Build an undirected graph > with
vertices [email protected]"99
A3B ; the last two are, respectively, the source and sink. Add edges :!C
@ , each
with weight D , and edges @'
AE , each with
weight C: . Finally, add F HG edges @I
@&( ,
each with weight "!#!$%&J
( . Then, cuts in >
are defined as follows:
Definition 1 A cut KILM of > is a partition of its
nodes into sets KONP?6!CBMQRKTS and LUNV?9A3BMQWLS ,
where !YX K[S\
AMX L]S . Its cost %H$&!9AKILM is the sum
Asymmetry is allowed, but we used symmetric scores.
ind1(Y) [.8]
ind2(Y) [.2]
assoc(Y,M) [1.0]
ind1(M) [.5]
assoc(Y,N) [.1]
ind2(M) [.5]
assoc(M,N) [.2]
ind2(N) [.9]
ind1(N) [.1]
Y,M a
` Y,M,N a
` Ya
` Na
` Ma
` Y,N a
M,N a
b leRbdhfegbji
b cfeRbdhfegb m
b legbdhfeRb m
b legbdhfeRbni
bdcfegbdhfegb m
b legbdhfegb m
bjikeRb c
ib oTeRbji
bnikeRb c
ib oTeRbdc
i6b ofeRb c
i6b ofeRbni
Figure 2: Graph for classifying three items. Brackets enclose example values; here,
^ _ the individual
^r scores happen to
be probabilities. Based on individual scores alone, we would put p (“yes”) in , q (“no”) in , and be undecided
about s (“maybe”). But the association scores favor cuts that put p and s in the same class, as shown in the table.
^ _
Thus, the minimum cut, indicated by the dashed line, places s together with p in .
of the weights of all edges crossing from K to
minimum cut of > is one of minimum cost.
Observe that every cut corresponds to a partition of
the items and has cost equal to the partition cost.
Thus, our optimization problem reduces to finding
minimum cuts.
Practical advantages As we have noted, formulating our subjectivity-detection problem in terms of
graphs allows us to model item-specific and pairwise information independently. Note that this is
a very flexible paradigm. For instance, it is perfectly legitimate to use knowledge-rich algorithms
employing deep linguistic knowledge about sentiment indicators to derive the individual scores.
And we could also simultaneously use knowledgelean methods to assign the association scores. Interestingly, Yu and Hatzivassiloglou (2003) compared an individual-preference classifier against a
relationship-based method, but didn’t combine the
two; the ability to coordinate such algorithms is
precisely one of the strengths of our approach.
But a crucial advantage specific to the utilization
of a minimum-cut-based approach is that we can use
maximum-flow algorithms with polynomial asymptotic running times — and near-linear running times
in practice — to exactly compute the minimumcost cut(s), despite the apparent intractability of
the optimization problem (Cormen, Leiserson, and
Rivest, 1990; Ahuja, Magnanti, and Orlin, 1993). 2
In contrast, other graph-partitioning problems that
have been previously used to formulate NLP classification problems3 are NP-complete (Hatzivassi2
Code available at http://www.avglab.com/andrew/soft.html.
Graph-based approaches to general clustering problems
are too numerous to mention here.
loglou and McKeown, 1997; Agrawal et al., 2003;
Joachims, 2003).
3 Evaluation Framework
Our experiments involve classifying movie reviews
as either positive or negative, an appealing task for
several reasons. First, as mentioned in the introduction, providing polarity information about reviews is a useful service: witness the popularity of
www.rottentomatoes.com. Second, movie reviews
are apparently harder to classify than reviews of
other products (Turney, 2002; Dave, Lawrence, and
Pennock, 2003). Third, the correct label can be extracted automatically from rating information (e.g.,
number of stars). Our data4 contains 1000 positive
and 1000 negative reviews all written before 2002,
with a cap of 20 reviews per author (312 authors
total) per category. We refer to this corpus as the
polarity dataset.
Default polarity classifiers We tested support vector machines (SVMs) and Naive Bayes (NB). Following Pang et al. (2002), we use unigram-presence
features: the th coordinate of a feature vector is
1 if the corresponding unigram occurs in the input
text, 0 otherwise. (For SVMs, the feature vectors
are length-normalized). Each default documentlevel polarity classifier is trained and tested on the
extracts formed by applying one of the sentencelevel subjectivity detectors to reviews in the polarity
Subjectivity dataset To train our detectors, we
need a collection of labeled sentences. Riloff and
Available at www.cs.cornell.edu/people/pabo/moviereview-data/ (review corpus version 2.0).
Wiebe (2003) state that “It is [very hard] to obtain collections of individual sentences that can be
easily identified as subjective or objective”; the
polarity-dataset sentences, for example, have not
been so annotated.5 Fortunately, we were able
to mine the Web to create a large, automaticallylabeled sentence corpus6 . To gather subjective
sentences (or phrases), we collected 5000 moviereview snippets (e.g., “bold, imaginative, and impossible to resist”) from www.rottentomatoes.com.
To obtain (mostly) objective data, we took 5000 sentences from plot summaries available from the Internet Movie Database (www.imdb.com). We only
selected sentences or snippets at least ten words
long and drawn from reviews or plot summaries of
movies released post-2001, which prevents overlap
with the polarity dataset.
Subjectivity detectors As noted above, we can use
our default polarity classifiers as “basic” sentencelevel subjectivity detectors (after retraining on the
subjectivity dataset) to produce extracts of the original reviews. We also create a family of cut-based
subjectivity detectors; these take as input the set of
sentences appearing in a single document and determine the subjectivity status of all the sentences
simultaneously using per-item and pairwise relationship information. Specifically, for a given document, we use the construction in Section 2.2 to
build a graph wherein the source ! and sink A correspond to the class of subjective and objective sentences, respectively, and each internal node @ corresponds to the document’s 'tvu sentence !6 . We can
set the individual scores 9:!/ to wyxC}/z|~{  :!/ and
:! to €‚ƒwyxC}z~{  :! , as shown in Figure 3,
where wyx }z~6{  :! denotes Naive Bayes’ estimate of
the probability that sentence ! is subjective; or, we
can use the weights produced by the SVM classifier instead.7 If we set all the association scores
to zero, then the minimum-cut classification of the
sentences is the same as that of the basic subjectivity detector. Alternatively, we incorporate the de5
We therefore could not directly evaluate sentenceclassification accuracy on the polarity dataset.
Available at www.cs.cornell.edu/people/pabo/moviereview-data/ , sentence corpus version 1.0.
We converted SVM output „ 8 , which is a signed distance
(negative=objective) from the separating hyperplane, to nonnegative numbers by
…v† „ E. ‡‰ˆ 8‰Š5‹JŽ
Œ: ’‘‡ ”)• „ 8 Š:–E—™„ ˜k”|š „ ; 8 š›” ;
„ 8 ˜k” .
˜ „ . ‡‰ˆ 8 Š . Note that scaling is employed
and „ 4 ‡‰ˆ 8 Š Ž
only for consistency; the algorithm itself does not require probabilities for individual scores.
gree of proximity between pairs of sentences, controlled by three parameters. The threshold L specifies the maximum distance two sentences can be
separated by and still be considered proximal. The
non-increasing function žT\" specifies how the influence of proximal sentences decays with respect to
distance ; in our experiments, we tried žT\ŸMNU€ ,
¡¢ , and €£# . The constant % controls the relative
influence of the association scores: a larger % makes
the minimum-cut algorithm more loath to put proximal sentences in different classes. With these in
hand8 , we set (for ¤¦¥§ )
¢E©Iª T
Ÿ!#!$%&:!J7!¨ ¬
N « ²ž ­¤‚g'k®6%
if ­¤¯g'°±L ;
4 Experimental Results
Below, we report average accuracies computed by
ten-fold cross-validation over the polarity dataset.
Section 4.1 examines our basic subjectivity extraction algorithms, which are based on individualsentence predictions alone. Section 4.2 evaluates
the more sophisticated form of subjectivity extraction that incorporates context information via the
minimum-cut paradigm.
As we will see, the use of subjectivity extracts
can in the best case provide satisfying improvement in polarity classification, and otherwise can
at least yield polarity-classification accuracies indistinguishable from employing the full review. At the
same time, the extracts we create are both smaller
on average than the original document and more
effective as input to a default polarity classifier
than the same-length counterparts produced by standard summarization tactics (e.g., first- or last-N sentences). We therefore conclude that subjectivity extraction produces effective summaries of document
4.1 Basic subjectivity extraction
As noted in Section 3, both Naive Bayes and SVMs
can be trained on our subjectivity dataset and then
used as a basic subjectivity detector. The former has
somewhat better average ten-fold cross-validation
performance on the subjectivity dataset (92% vs.
90%), and so for space reasons, our initial discussions will focus on the results attained via NB subjectivity detection.
Employing Naive Bayes as a subjectivity detector (Extract NB ) in conjunction with a Naive Bayes
document-level polarity classifier achieves 86.4%
Parameter training is driven by optimizing the performance
of the downstream polarity classifier rather than the detector
itself because the subjectivity dataset’s sentences come from
different reviews, and so are never proximal.
Pr (s1)
¹ º¹
¹ sº¹
¶ µ¶
¶ v1µ¶
min. cut
· ¸·
¸· s¸·
m−sentence extract
n−sentence review
individual subjectivity−probability link
edge crossing the cut
proximity link
Figure 3: Graph-cut-based creation of subjective extracts.
accuracy.9 This is a clear improvement over the
82.8% that results when no extraction is applied
(Full review); indeed, the ² difference
is highly sta²
€ , paired t-test). With
tistically significant (»½¼
SVMs as the polarity classifier instead, the Full review performance rises to 87.15%, but comparison
via the paired t-test reveals that this is statistically
indistinguishable from the 86.4% that is achieved by
running the SVM polarity classifier on Extract NB
input. (More improvements to extraction performance are reported later in this section.)
These findings indicate10 that the extracts preserve (and, in the NB polarity-classifier case, apparently clarify) the sentiment information in the originating documents, and thus are good summaries
from the polarity-classification point of view. Further support comes from a “flipping” experiment:
if we give as input to the default polarity classifier
an extract consisting of the sentences labeled objective, accuracy drops dramatically to 71% for NB
and 67% for SVMs. This confirms our hypothesis
that sentences discarded by the subjectivity extraction process are indeed much less indicative of sentiment polarity.
Moreover, the subjectivity extracts are much
more compact than the original documents (an important feature for a summary to have): they contain
on average only about 60% of the source reviews’
words. (This word preservation rate is plotted along
the x-axis in the graphs in Figure 5.) This prompts
us to study how much reduction of the original documents subjectivity detectors can perform and still
accurately represent the texts’ sentiment information.
We can create subjectivity extracts of varying
This result and others are depicted in Figure 5; for now,
consider only the y-axis in those plots.
Recall that direct evidence is not available because the polarity dataset’s sentences lack subjectivity labels.
lengths by taking just the ¾ most subjective sentences11 from the originating review. As one baseline to compare against, we take the canonical summarization standard of extracting the first ¾ sentences — in general settings, authors often begin documents with an overview. We also consider the last ¾ sentences: in many documents,
concluding material may be a good summary, and
www.rottentomatoes.com tends to select “snippets”
from the end of movie reviews (Beineke et al.,
2004). Finally, as a sanity check, we include results
from the ¾ least subjective sentences according to
Naive Bayes.
Figure 4 shows the polarity classifier results as
¾ ranges between 1 and 40. Our first observation
is that the NB detector provides very good “bang
for the buck”: with subjectivity extracts containing
as few as 15 sentences, accuracy is quite close to
what one gets if the entire review is used. In fact,
for the NB polarity classifier, just using the 5 most
subjective sentences is almost as informative as the
Full review while containing on average only about
22% of the source reviews’ words.
Also, it so happens that at ¾¿NÁÀ , performance
is actually slightly better than (but statistically indistinguishable from) Full review even when the
SVM default polarity classifier is used (87.2% vs.
87.15%).12 This suggests potentially effective extraction alternatives other than using a fixed probability threshold (which resulted in the lower accuracy of 86.4% reported above).
Furthermore, we see in Figure 4 that the ¾ most11
These are the  sentences assigned the highest probability
by the basic NB detector, regardless of whether their probabilities exceed 50% and so would actually be classified as subjective by Naive Bayes. For reviews with fewer than  sentences,
the entire review will be returned.
Note that roughly half of the documents in the polarity
dataset contain more than 30 sentences (average=32.3, standard
deviation 15).
Accuracy for N-sentence abstracts (def = SVM)
Average accuracy
Average accuracy
Accuracy for N-sentence abstracts (def = NB)
N most subjective sentences
last N sentences
first N sentences
N least subjective sentences
Full review
N most subjective sentences
last N sentences
first N sentences
N least subjective sentences
Full review
Figure 4: Accuracies using N-sentence extracts for NB (left) and SVM (right) default polarity classifiers.
Accuracy for subjective abstracts (def = NB)
Accuracy for subjective abstracts (def = SVM)
Average accuracy
Average accuracy
difference in accuracy
not statistically significant
Full Review
ExtractNB ExtractSVM+Prox
difference in accuracy
not statistically significant
indicates statistically significant
improvement in accuracy
% of words extracted
Full Review
indicates statistically significant
improvement in accuracy
% of words extracted
Figure 5: Word preservation rate vs. accuracy, NB (left) and SVMs (right) as default polarity classifiers.
Also indicated are results for some statistical significance tests.
subjective-sentences method generally outperforms
the other baseline summarization methods (which
perhaps suggests that sentiment summarization cannot be treated the same as topic-based summarization, although this conjecture would need to be verified on other domains and data). It’s also interesting
to observe how much better the last ¾ sentences are
than the first ¾ sentences; this may reflect a (hardly
surprising) tendency for movie-review authors to
place plot descriptions at the beginning rather than
the end of the text and conclude with overtly opinionated statements.
Incorporating context information
The previous section demonstrated the value of
subjectivity detection. We now examine whether
context information, particularly regarding sentence
proximity, can further improve subjectivity extraction. As discussed in Section 2.2 and 3, con-
textual constraints are easily incorporated via the
minimum-cut formalism but are not natural inputs
for standard Naive Bayes and SVMs.
Figure 5 shows the effect of adding in
proximity information.
Extract NB+Prox and
ExtractSVM+Prox are the graph-based subjectivity
detectors using Naive Bayes and SVMs, respectively, for the individual scores; we depict the
best performance achieved by a single setting of
the three proximity-related edge-weight parameters
over all ten data folds13 (parameter selection was
not a focus of the current work). The two comparisons we are most interested in are Extract NB+Prox
versus ExtractNB and ExtractSVM+Prox versus
ExtractSVM .
We see that the context-aware graph-based subjectivity detectors tend to create extracts that are
4 are chosen from ÃÅÄÇÆ È ” ÈÉHÊ , Ë ‡ „ Š Ä
Æ ‘3È/Ì ‹ IÈ ‘ – „ Ê , and ÎkÄÐÏ œ È
‘JÑ at intervals of‘30.1.
more informative (statistically significant so (paired
t-test) for SVM subjectivity detectors only), although these extracts are longer than their contextblind counterparts. We note that the performance
enhancements cannot be attributed entirely to the
mere inclusion of more sentences regardless of
whether they are subjective or not — one counterargument is that Full review yielded substantially
worse results for the NB default polarity classifier—
and at any rate, the graph-derived extracts are still
substantially more concise than the full texts.
Now, while incorporating a bias for assigning
nearby sentences to the same category into NB and
SVM subjectivity detectors seems to require some
non-obvious feature engineering, we also wish
to investigate whether our graph-based paradigm
makes better use of contextual constraints that can
be (more or less) easily encoded into the input of
standard classifiers. For illustrative purposes, we
consider paragraph-boundary information, looking
only at SVM subjectivity detection for simplicity’s
It seems intuitively plausible that paragraph
boundaries (an approximation to discourse boundaries) loosen coherence constraints between nearby
sentences. To capture this notion for minimum-cutbased classification, we can simply reduce the association scores for all pairs of sentences that occur in different paragraphs by multiplying² them by
ZÔÓ 9€¨Õ . For
a cross-paragraph-boundary weight Ò
standard classifiers, we can employ the trick of having the detector treat paragraphs, rather than sentences, as the basic unit to be labeled. This enables the standard classifier to utilize coherence between sentences in the same paragraph; on the other
hand, it also (probably unavoidably) poses a hard
constraint that all of a paragraph’s sentences get the
same label, which increases noise sensitivity. 14 Our
experiments reveal the graph-cut formulation to be
the better approach: for both default polarity classifiers (NB and SVM), some choice of parameters
(including Ò ) for Extract SVM+Prox yields statistically significant improvement over its paragraphunit non-graph counterpart (NB: 86.4% vs. 85.2%;
SVM: 86.15% vs. 85.45%).
5 Conclusions
We examined the relation between subjectivity detection and polarity classification, showing that subjectivity detection can compress reviews into much
shorter extracts that still retain polarity information
at a level comparable to that of the full review. In
For example, in the data we used, boundaries may have
been missed due to malformed html.
fact, for the Naive Bayes polarity classifier, the subjectivity extracts are shown to be more effective input than the originating document, which suggests
that they are not only shorter, but also “cleaner” representations of the intended polarity.
We have also shown that employing the
minimum-cut framework results in the development of efficient algorithms for sentiment analysis. Utilizing contextual information via this framework can lead to statistically significant improvement in polarity-classification accuracy. Directions
for future research include developing parameterselection techniques, incorporating other sources of
contextual cues besides sentence proximity, and investigating other means for modeling such information.
We thank Eric Breck, Claire Cardie, Rich Caruana,
Yejin Choi, Shimon Edelman, Thorsten Joachims,
Jon Kleinberg, Oren Kurland, Art Munson, Vincent
Ng, Fernando Pereira, Ves Stoyanov, Ramin Zabih,
and the anonymous reviewers for helpful comments.
This paper is based upon work supported in part
by the National Science Foundation under grants
ITR/IM IIS-0081334 and IIS-0329064, a Cornell
Graduate Fellowship in Cognitive Studies, and by
an Alfred P. Sloan Research Fellowship. Any opinions, findings, and conclusions or recommendations
expressed above are those of the authors and do not
necessarily reflect the views of the National Science
Foundation or Sloan Foundation.
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